1. Field of the Invention
This invention relates to the resistance welding apparatus for welding for example the longitudinal seams of the bodies of containers such as are conventionally used for packaging food and aerosol products. In particular, but not exclusively the invention is concerned with resistance welding apparatus of the kind having electrodes in the form of opposed wheels between which may be passed the overlapped margins of a metal body blank bent to tubular form, and to which electric current may be supplied to weld the margins together as the required body seam.
2. Description of the Prior Art
In the past, various attempts have been made to monitor the weld achieved by such an apparatus as it is being formed, so as to enable container bodies with substandard welds to be rejected, to enable one or more parameters of the welding process to be controlled by closed or open loop control, and/or for data logging or indication purposes. In our British patent specifications No. 2083235B and 2139361B there is described and claimed one such monitoring system, in which the desired signal is derived from repeated integration of the weld voltage during welding.
Although representing a considerable advance over the resistance weld monitoring systems available hitherto the systems described in the above-mentioned British patent specifications gave only a partial picture of the welding process, and attempts have since been made to use other parameters in addition as an indication of weld quality.
Known apparatus for resistance welding the seams of can bodies are the welding machine marketed by Soudronic AG under their series designation FBB. In those machines two electrode wheels are mounted one above the other as a cooperating pair, the upper wheel being driven by an associated motor.
The blank margins destined to form the welded seam are overlapped with one another and presented to the nip between the electrode wheels by what is commonly referred to as a "Z-bar" because of its Z shaped cross-sectional profile. The Z-bar and the lower electrode wheel are mounted virtually immovably in relation to one another from the machine frame; the upper electrode wheel, however, is vertically immovable and biassed downwardly by a compression spring so that the upper electrode wheel exerts a substantially constant welding pressure on the formative seam as welding proceeds.
In an FBB series machine potentially one of the most useful parameters to use as an indication of weld quality is the vertical position of the upper electrode wheel, since any variation in the thickness of the can body seam must cause a substantially equal vertical bodily movement of that wheel.
An attempt to sense the position of the axis or shaft of the upper electrode wheel, by means of a transducer sensing the movement of the axis and producing an electrical signal indicative of position, gave inaccurate measurements of weld thickness. This was because eccentric movement of the wheel relative to the axis produced a signal component which masked the signal component representative of variations in weld thickness. This prior art method is therefore limited in value.
Applicants believe that by suitable choice of methods and apparatus a signal can be derived which is more accurately representative of the weld thickness than has hiterto been possible.